They state four general reasons why evaluation results are often not used:

Fail to focus on intended use by intended users and are not designed to fit the context and situation

Do not focus on the most important issues – resulting in low relevance

Are poorly understood by stakeholders

Fail to keep stakeholders informed and involved during the process and when design alterations are necessary.

I think the first and last reasons are particularly pertinent. We often don’t have enough insights into how evaluation results will be used – and we also fail to inform and involve stakeholders during the actual evaluation.